This invention relates to pneumnatic tires and particularly to radial type pneumatic tires.
Radial type pneumatic tires typically include a crown area with an annular belt, a pair of sidewall areas, a pair of bead portions, and a carcass or body which includes one or more carcass plies of rubberized cords oriented substantially at a 90.degree. angle (radially) to the circumferential center plane of the tire. The cords of these carcass plies can be made of such material as metal, glass; natural textile or synthetic textile such as rayon, nylon, polyester, aromatic polyamide or other similar materials.
The carcass of a radial tire provides structural stability to the tire reduces susceptibility of the tire to bruising, and reduces the possibility of separation of tire components occurring within the tire. In the crown area of the tire, an annular belt also aids in reducing susceptibility of the tire to bruising.
The number of carcass plies incorporated into a radial tire carcass is usually dependent on the intended service or use of the tire. Radial tires having at least two continuous carcass plies include such tires serving as large passenger tires and light truck radial tires. In such tires, the plies generally extend continuously from one bead portion of the tire adjacent to the shoulder and crown areas to the other bead portion and in most tires each ply is wrapped around a wire bead in each bead portion.
It has been found in a radial tire carcass with two continuous inner (first) and outer (second) plies that the second ply provides limited structural stability in the crown area of the tire and the bead portions of the tire. The second ply also provides limited resistance to bruising in the crown area of the tire because the belt provides much of the resistance to bruising.
Tire manufacturers are continuously searching for ways to reduce the overall weight of radial tires and the costs to manufacture them. Relatively unsuccessful attempts have been made to design a single ply radial tire for service which traditionally requires two or more carcass plies. Other attempts have focused on reducing the amount of carcass material in one of the plies of a multi ply tire, as illustrated in tires containing discontinuous radial outer or second plies as described, for example, in U.S. Pat. No. 3,058,509; 3,062,259; 3,481,386; 3,509,930; 4,096,899; 4,185,675; 4,341,250; 4,573,511 and European Patent Number 0,161,202. In '509, '386, '930, and '899 tire are disclosed each having an outer carcass ply which is discontinuous or interrupted in the crown area of the tire, but is out of contact with the inner carcass ply in the bead area.
The outer plies disclosed in '386; '930 and '899, for example, are also discontinuous in the crown area of tires as well as at locations adjacent to the bead wire. Often the cords and/or rubber in such a discontinuous outer ply are of special materials other than the materials that make up the continuous radial inner or first ply.
The tires disclosed in '259 and '250 do not utilize split plies while '202 employs two plies, both of which are split.
In '675, for example, a second discontinuous ply is different from the first ply in that the rubber of the second ply has a shore A hardness after vulcanization of 40.degree. to 50.degree. . Weight and cost advantages of incorporating a discontinuous ply are often negated when the materials in the second discontinuous ply are different from the materials in the first continuous ply.
The split ply tires of '675 and '511 do not terminate their outer carcass ply and liner at the bead and, like all of the prior art, do not employ two identical carcass plies in continuous contact with each other and both with radial cords.
Also, it has been found that significant forces can act between the rim of the wheel upon which the tire is mounted and the lower sidewall portions of the tire. In the sidewall portion, significant forces act on the tire in the region where the wheel rim flange physically contacts the sidewall therein. In this rim flange contact region, the sidewall flexes over the rim flange. The rim flange contact region of the tire must be made strong enough to endure such flexing and forces resulting therefrom.
Furthermore, it has been found that a predetermined thickness in each bead portion between the radial inner part of the bead wire and the surface of the bead portion must be maintained to allow for suitable mounting of the tire on the rim.
Thus, when designing tires with discontinuous carcass plies, unsatisfactory tires can result by ignoring particular areas of the tire where special forces occur and specific requirements in the bead area are desired.